Update on MiniBooNE H. A. Tanaka a a Department of Physics, Joseph - - PDF document

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Update on MiniBooNE

• H. A. Tanaka a

aDepartment of Physics, Joseph Henry Laboratories

Princeton University Princeton, New Jersey 08544, USA

MiniBooNE (Booster Neutrino Experiment) is searching for νµ → νe oscillations in the neutrino beam produced by the 8 GeV Booster synchrotron at Fermilab. The Booster has delivered 3.66×1020 protons-on-target with over 380 thousand neutrino recorded in the detector since September 2002. MiniBooNE is now accumulating enough data to achieve its goal of conclusively confirming or refuting the evidence for neutrino oscillations observed by the LSND experiment.

• 1. Introduction

The MiniBooNE detector is a 610 cm ra- dius sphere filled with mineral oil instrumented with photomultipliers. The detector is divided into two optically isolated concentric regions; an

• uter veto region with 240 photomultipliers and

an inner “tank” region with 1280 photomultipli- ers. Neutrino interactions are detected via the Cherenkov radiation and scintillation light pro- duced by charged particles passing through the mineral oil. The veto detects charged particles entering or exiting the tank region and is used to reject cosmic muons and select contained neu- trino interactions. The neutrino beam is produced by protons from the 8 GeV Booster synchrotron at FNAL. At design intensity, 5×1012 protons are extracted to the MiniBooNE beamline in a 1.6 µ sec pulse at a rate of 5 Hz. The beam is incident on a beryllium target inserted inside a magnetic horn, where secondary pions and kaons are produced and focussed into the 50 meter-long decay region. The subsequent decay of the secondary particles produce a nearly pure νµ beam, with average en- ergy of 800 MeV and O(10−3) νe contamination. The small νe content is important for the sensi- tive search for νµ → νe oscillation that is the goal

• f the experiment. The expected neutrino energy

distribution at the detector is shown in Figure 1. In this energy range, the cross section for neu- trino interactions are dominated by the charged current quasi-elastic interaction (CCQE), which comprise about 40% of the events. Neutral cur- rent elastic scattering and resonant single pion production (both neutral and charged current) comprise nearly the rest.

• 2. Physics

The primary physics goal of MiniBooNE is to confirm the evidence for νµ → νe oscillations ob- served by the LSND experiment [1]. The evidence suggests a ∆m2 ranging from 10−1−101 eV2, with ∼ 0.25% oscillation probability. The 540 meter distance of the detector from the target is chosen to reproduce the L/E distribution of the νe excess in the LSND experiment (∼ 1m/ MeV) and max- imize the sensitivity of the experiment to these

• scillations.

The phenomena of neutrino oscillations, consid- ered speculative only a decade ago, are now definitively established in two modes: the “solar” νe → νx oscillations with ∆m2 ∼ 8 × 10−5 eV2 and large (but not maximal) mixing [2][3][4][5] [6], and the “atmospheric” νµ → νx oscillations with ∆m2 ∼ 2.5 × 10−3 eV2 and maximal mixing [7][8][9]. The evidence for

• scillations

reported by LSND, however, remains unconfirmed by other experiments. Its place in the phenomenology

• f neutrino oscillations is intriguing, since the

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0.5 1 1.5 2 2.5 3 Eν (GeV) Fraction of ν Flux / 0.1 GeV

ν Flux νe Flux

µ µ

Figure 1. Energy distribution of neutrinos at MiniBooNE obtained from Monte Carlo simula- tion. three known active flavors of neutrinos are unable to simultaneously accommodate the three ∆m2 regimes observed by the experiments. A dras- tic remedy to the Standard Model (minimally ex- tended to include neutrino oscillations) is needed if the phenomenon is confirmed.

• 3. Identifying νe Events in MiniBooNE

MiniBooNE will search for νµ → νe oscillations by detecting an excess of νe CCQE interactions in the detector. This signal process is identified by the spatial and temporal distribution of hits reconstructed in the photomultiplier array. For example, the muons emerging from νµ CCQE in- teractions produce Cherenkov ring distributions with cleaner edges than the electrons from νe CCQE interactions, which undergo multiple scat- tering in the medium. The primary backgrounds to the oscillation νe CCQE interactions take two forms. The first are misidentified νµ events, primarily neutral current π0 events. The photons emerging from the decay

• f the π0 convert in the medium, producing an

e+e− pair. If the photons are highly asymmetric in energy or have small opening angle, the pho- tons will appear much like the primary electron emerging from a νe CCQE interaction. Other- wise, the two-ring topology of these events can be used to reject them. The particle algorithms incorporate parameters describing the ring sharp- ness and the overall profile to reject events with muon-like rings and multiple rings. A second class of backgrounds come from interac- tions of νe intrinsically present in the beam from the decay of muons and kaons in the decay re-

• gion. This background is irreducible; it cannot

be distinguished in any way from the signal pro- cess apart from its overall energy distribution. The expected signal and background rates, as- suming ∆m2 = 1 eV2 and sin2 2θ = 0.002 and 1021 protons-on-target are shown in Table 1 with and without selection. The expected sensitivity using a fit to the energy spectrum of the expected events is shown in Figure 2.

• 4. Systematic Studies

In order to estimate reliably both classes of background in the analysis, a detailed under- standing of both the neutrino beam and the de- tector performance is needed. A two-prong effort using offline measurements to complement detec- tor data is in place to develop this understanding and reduce the systematic uncertainties. For the misidentified νµ events, a precise esti- mate of the event rates in the detector and an accurate simulation of the detector response to these interactions is necessary. This allows accu- rate predictions of the performance of the particle identification algorithms used to identify the sig- nal process. The experiment is currently under- taking a detailed study of the detector behavior using the detector calibration systems. Neutrino interactions observed in the detector, including the background νµ-induced π0 production, are also being analyzed [10]. These in situ efforts are complemented by ex situ measurements of oil op- tical properties, including scattering, fluorescence and scintillation measurements [11]. MiniBooNE collaborators are active in the HARP experiment, where the kaon production rates needed to estimate the intrinsic νe back- ground are being measured [12]. The latter mea- surements are cross-checked by the Little Muon Counter (LMC), a spectrometer that measures

3 Process All Events After Selection νµ CCQE 553 × 103 8 νµ NC π0 110 × 103 290 νµ ∆ → (n/p)γ 1 × 103 80 Intrinsic νe 2.5 × 103 350 Oscillation signal 1.5 × 103 300 Table 1 The expected event yields for the primary back- ground channels and the oscillation signal with ∆m2 = 1 eV2 and sin2 2θ = 0.002 and 1021 protons-on-target. the spectrum and rate of wide-angle muon pro- duction resulting from kaon decay in the 50 meter decay region[13].

• 5. Current Status and Outlook

The MiniBooNE detector has recorded nearly four hundred thousand neutrino interactions since data-taking began in 2002. During this time, the Fermilab Booster has delivered 3.66 × 1020 pro- tons to the beryllium target used to produce the neutrino beam. The experiment is now accumu- lating the 1021 protons-on-target needed to make a conclusive confirmation or refutation of the LSND evidence for neutrino oscillations, while completing systematic studies necessary for reli- ably estimating signal efficiencies and background rates.

• 6. Acknowledgments

The collaborating institutions thank the Fermi National Accelerator Laboratory (FNAL) for its kind hospitality. We are grateful for the excel- lent performance provided by the Accelerator Di- vision and the strong support of the Directorate

• f FNAL. This work is funded by the Department
• f Energy and the National Science Foundation of

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